The present invention relates to a signal processing technique for a video signal processor including a color television camera, and particularly relates to an apparatus and a method of registration discrepancy correction suitable to a television camera used in a system in which image pickup elements are joined to a color separation optical system, and a television camera having a registration correcting function.
So-called three-chip color television cameras are hitherto prevailed as television cameras used for applications such as broadcasting applications in which comparatively high performance specifications are required. The three-chip color television cameras adopt a system in which an incident image is separated into images of the three primary colors R (Red), G (Green) and B (Blue) by a prism, and the color images are transformed into electric signals by solid state image pickup elements such as CCDs respectively.
Here, FIG. 7 shows an example of such a three-chip television camera. In FIG. 7, an image obtained by a not-shown optical lens system is separated into images of the three primary colors R, G and B by a color separation prism 1. The color images are transformed into electric signals by solid state image pickup elements 2R, 2G and 2B of their corresponding color channels, respectively, and then supplied to an analog signal processing circuit 3.
Then, the electric signals subjected to processing such as amplification by the analog signal processing circuit 3 are converted into digital signals by an AD conversion circuit 4. Next, the digital signals are subjected to processes such as gamma correction, edge enhancement, and so on, by a digital signal processing circuit 5 so as to be formed as output signals.
Such a three-chip television camera typically adopts a system in which the respective solid state image pickup elements 2R, 2G and 2B are semi-permanently fixedly stuck to the color separation prism 1 by an adhesive agent or the like. In this case, however, when optical alignment of the three solid state image pickup elements is carried out inaccurately, the positions of the elements cannot be re-adjusted to eliminate the displacement.
This results in so-called registration discrepancy in which images based on three color signals are not perfectly matched to one another when they are reproduced. The registration discrepancy leads to the deterioration of the resolution, the generation of false color signals, and so on. Thus, the image quality of a reproduced image is deteriorated on a large scale. Therefore, high accuracy is required for positioning the solid state image pickup elements when they are stuck to the prism.
In addition, the high-definition television age has opened in recent years so that the number of pixels in image pickup elements has increased dramatically. Therefore, the required accuracy of positioning is extremely high so that advanced technology and extensive time are required for the positioning work. In addition, since even slight deformation gives a great influence, it is extremely difficult to keep the performance after manufacturing.
As for horizontal positioning of image pickup elements, delay properties differing among the R, G and B color channels in a video signal circuit system or a signal transmission system such as signal lines also cause a registration discrepancy. Accordingly, the horizontal positioning has to be carried out with higher precision in consideration of such a registration discrepancy. Thus, it is more and more difficult to obtain high precision.
Therefore, a method in which suppression of registration discrepancies depends on not only the assembling accuracy of an optical system but also electric correction based on processing of image signals has been hitherto known. For example, in the prior art, there is a method as follows. That is, delay times or phases among respective color signals are adjusted by analog delay lines before the color signals are AD-converted. Thus, registration discrepancies are corrected.
The aforementioned prior-art technique gives no consideration to the influence of use of the analog delay lines. Thus, there are problems that the cost increases and the performance cannot be kept. That is, since it is necessary to add the analog delay lines, extra wiring and extra work are required so as to cause a lot of labor and time. Thus, the cost increases. In addition, since the delay properties using the analog delay lines are fixed, a variety of discrepancies cannot be dealt with flexibly, and a variety of properties cannot be dealt with, either. Thus, there also arises a problem that the performance cannot be kept.
Japanese Patent No. 2744577 issued to Yamamoto et al discloses a method in which, when a color signal is read from each of CCD elements of the colors R, G and B, the phase of a reference horizontal transfer pulse is adjusted on the basis of a registration error by a delay circuit so as to correct the registration. According to this method, since a plurality of horizontal transfer pulses having different frequencies are produced, the circuit configuration as a whole becomes complicated, and the adjustment is also troublesome.
U.S. Pat. No. 5,113,247 issued to Akiyama et al and JP-A-6-303622 disclose that a two-dimensional filter is used chiefly to eliminate color shift caused by lens aberration. The correction of chromatic aberration using the two-dimensional filter increases the circuit scale so that the cost increases.